Various methods have been devised for enabling the safe continued operation of un-pressurized or under pressurized vehicle tires with the intent of minimizing further damage to the uninflated tire and without compromising vehicle handling while driving to where the tire can be changed. Loss of tire pressure can result from a variety of causes, including puncture by a foreign object such as a nail. Pneumatic tires designed for sustained operation when “uninflated” (“flat”) or under inflated are also called “runflat” tires, as they are capable of being driven in the “flat” condition. They are also called extended mobility technology tires, or “EMT” tires. A conventional pneumatic tire will collapse upon itself when it is uninflated and carrying the weight of a vehicle. The tire's sidewalls buckle outward in the circumferential portion of the tire where the tread contacts the ground, making the tire “flat.”
The term “runflat” is generally used to describe a tire that is designed such that the tire structure alone, and in particular the structure of the sidewalls, has sufficient strength and rigidity to support the vehicle load when the tire is operated without being inflated. The sidewalls and internal surfaces of such runflat tires or EMT tires do not collapse or buckle due to their rigidity, and the prevailing designs of such tires do not otherwise contain or use other supporting structures or devices to prevent the tire from collapsing. Examples of such other supporting structures are devices that might be contained within the tire and which experience no loading during normal inflated operation.
In general, runflat tires or EMT tires incorporate sidewalls that are thicker and/or stiffer so that the tire's load can be carried by an uninflated tire with minimum adverse effects upon the tire itself and upon vehicle handling until such reasonable time as the tire can be repaired or replaced. The typical methods used in sidewall thickening and stiffening include the incorporation of circumferentially disposed wedge inserts in the inner peripheral surface of the sidewall portion of the carcass, which is the region in the tire usually having the lowest resistance to deformation under vertical loading. In such runflat tire designs, each sidewall is thickened in the region between the bead and the tread shoulder. The wedge inserts in each sidewall are generally crescent-shaped in cross-sectional view, in order to conform to the shape of the sidewalls. Such wedge reinforced sidewalls, when operated in the uninflated condition, experience a net compressive load in the region of the sidewall that is closest to the road-contacting portion of the tread. More specifically, the bending stresses on the sidewalls are such that the axially outwardmost portions of the reinforced sidewalls experience tensile stresses while the axially inward portions experience compressive stresses during runflat operation.
A Goodyear patent, U.S. Pat. No. 5,368,082 ('082), by Oare et al, discloses a low aspect ratio runflat pneumatic radial ply tire which employs multiple wedge inserts in each sidewall to improve runflat stiffness. Approximately six additional pounds of weight per tire was required to support an 800 lb. load in this uninflated tire. This earlier invention, although superior to prior attempts at runflat tire design, still imposed a weight penalty which could, however, be partially offset by the elimination of a spare tire and the tire jack. However, this weight penalty becomes even more problematic in the design of tires having higher aspect ratios. The '082 patent teaches a sidewall construction for runflat tires in which the tire is constructed with two plies, an inner liner and two reinforcing wedge inserts in each sidewall. The two inserts in each sidewall are disposed such that one insert is located between the two plies while the other insert is located between the inner liner and the first or innermost ply.
Two U.S. Pat. Nos. 5,427,166 and 5,511,599 of Walter L. Willard, Jr., show Michelin tires that incorporate an additional third ply and a third insert in the sidewall to further increase the runflat performance of the tire over that of the '082 patent. These Willard patents discuss some of the load relationships that occur in the uninflated condition of the tire and demonstrate that the concept taught in the '082 patent can be applied to additional numbers of plies as well as additional wedge inserts in each sidewall.
However, such large amounts of rubber used to stiffen the sidewall members become factors in flexure heating that leads to tire failure during runflat operation. This is especially so when the tire is operated at high speeds during low or zero inflation. Therefore, one goal of runflat tire design is to minimize the number of wedge inserts used to stiffen each sidewall and the total amount of wedge insert material used in runflat tire.
While the high resistance to compression and deflection of the inserts provides the necessary resistance to the collapse of the uninflated loaded tire, the use of multiple plies and more than one reinforcing wedge insert in each sidewall has drawbacks which include the above mentioned increase in tire weight and flexure-induced heat buildup. Such designs also increase the tire's complexity in ways that adversely affect manufacturing and quality control.
Although many of the run-flat tire constructions set forth in the above-referenced patents have proven to be successful for certain applications, these constructions are usually found in low profile tires, that is, applications in which the tires have an aspect ratio of not more than 50%. These tires are of the type usually found on high performance vehicles. It is somewhat more difficult to implement run-flat tire constructions for higher profile tires, that is tires having an aspect ratio of greater than 50%, so that the tires have both sufficient uninflated durability and good subjective ride performance in the inflated condition.
The following patents are also noted as being of interest.
U.S. Pat. No. 3,911,987 (Takusagawa, et al.) discloses a pneumatic safety tire for motorcycles which includes a pair of elastic reinforcing layers disposed along the carcass plies of the tire and extending from proximal the tire beads to a tire inner surface under the tread rubber. The Shore A hardness of the reinforcing layer is 45 or more, so that the safety tire can run at a high speed even after puncture, while providing excellent handling characteristics under normal conditions.
U.S. Pat. No. 4,203,481 (Ranik, Jr.) discloses a pneumatic tire, rim, and a combination thereof that yields improved stability characteristics when the tire is run flat. The tire has an asymmetric sidewall construction resulting from the inclusion of rubber inserts in its sidewalls of different bulk (thickness) than one another, and the rim has axially outward extending rim flange extensions which are angled in relation to the axis of rotation of the rim. The rubber inserts (13, 14) are located inwardly of the carcass reinforcing material or plies of the tire. The inserts are generally crescent-shaped, and are located at the midpoint of the sidewall; that is, the distance half way between the bead seat and the road engaging tread surface when the tire is mounted and inflated under normal conditions. The inserts are located inside the reinforcing body ply. The inserts in the vehicle side are 0.25 inches (0.63 cm) thick on one side of the tire and 0.20 inches (0.51 cm) thick on the other side of the tire. The modulus of the rubber compound utilized in the inserts is 1300 psi at 200% elongation.
U.S. Pat. No. 4,265,288 (Kaneko, et al.) discloses a pneumatic safety tire having annular rubber reinforcements (sidewall inserts) having crescent sectional shape and applied to a tire carcass at tire sidewalls, the rubber of the reinforcements having a JIS hardness of not less than 70, a tensile stress (Mod25) after an aging test of not less than 10 kg/cm2, and a repulsive elasticity by Dunlop tripsometer of not less than 65%.
U.S. Pat. No. 5,769,980 (Spragg, et al.) discloses a pneumatic passenger tire having an aspect ratio of greater than 50% and having crescent-shaped sidewall inserts.
U.K. Patent Application No. GB 2,087,805 discloses a pneumatic safety tire having at least two carcass ply structures (38,40), and disposed adjacent the radially inner surface of each ply structure (38,40) there is an annular elastomeric insert (42,46). One insert (42) extends from the bead region radially outward, terminating beneath the tread reinforcing belt structure (36), preferably a distance from the tread edge a distance A of at least ten percent (10%) and not greater than forty percent (40%) of the tread width, such as approximately 25% of the tread width. The other insert (46) is disposed between reinforcing ply structures (38,40), and extends from the bead region radially outward, terminating beneath the tread reinforcing belt structure (36) in the same manner as the one insert (42). The elastomeric inserts (42) each have a thickness (B), a the maximum section diameter of the tire of at least one percent (1%), preferably at least 3% and not greater than 5% of the maximum section diameter of the tire. (The maximum section diameter of the tire is measured parallel to the rotational axis of the tire from the axially outer surfaces of the tire, exclusive of indicia, adornment and the like.) Each elastomeric insert (46) has a thickness C of at least one percent (1%), preferably in the range of 2-4%, and not greater than 5%, of the maximum section diameter of the tire.
Clearly, the goal in runflat tire design is to provide a low-cost, light-weight tire that gives both good runflat vehicle handling as well as good service life during runflat operation, while providing excellent handling characteristics under normal operating conditions.